Breakfast in bed is when one wakes up next to (or happens to come upon) a sleeping lady companion and proceeds to perform cunnilingus on her. This is a gennerally appreciated method of being woken up. The phrase makes allusions to the term "eating out" as the person performing the oral ministrations is having, euphemisticaly, their first meal of the day in bed. In this case the meal happens to be a woman's genetalia and there is still a need to eat breakfast afterward.
This is similar to, but differs in key ways from, a mongoose.

I had great sex with Tiffany last night, so when I woke up and saw that she was still asleep, I had breakfast in bed to thank her.

Lisa I have to call you back, Frank is having breakfast in bed with me. What is he eating? Uh...